The average time delay in electromagnetic scattering by a spherically symmetric, reflection-invariant, nonabsorbing scatterer is evaluated, extending results previously derived for scalar nonrelativistic scattering. For the lth partial wave in each polarization, the time delay is the spectral average of dηₗ/d{ω} over the incident wave packet (where ηₗ is the corresponding phase shift and {ω} the angular frequency). This is one-half of the Eisenbud-Wigner nonrelativistic expression. The relationship with angular time delay in a given direction is also discussed.
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H. M. Nussenzveig (1997) studied this question.
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